Knowledge + experience = understanding. Learning means getting an understanding.
If you know something but cannot apply in real-world case you do not understand it. Knowledge is useless unless you can apply it.
Knowledge without understanding is like luggage, something external you carry with you. It is easy to lose. Understanding becomes a part of you. So it is difficult to lose what you have understood.
You can find virtually any knowledge on the internet in 5 minutes, but unless it is something trivial it is useless until you have an experience applying it.
You do not want to study only to forget it all soon. It means you should learn something only if you can start to apply it right away. If you do not have the opportunity to apply your knowledge, do not collect too much of it. Knowledge without real experience is like heavy luggage you are going to lose anyway.
In the book “Stranger in a strange lang” Robert Heinlein invented an interesting word “grok” to emphasize a deep understanding. If you need to learn something you have to grok it.
Understanding has many levels. A kid may understand how a computer works like this: When I press a “power” button and then click the game icon the computer is going to start my game. If the kid stops on that level of understanding he is not going to be a good IT specialist 😉